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In recirculating aquaculture system (RAS), ammonium excreted by the fish is typically transformed to less toxic nitrate by microbial activity in bioreactors. However, nitrate-nitrogen load can be harmful for the receiving water body when released from the RAS facility. A new type of water treatment system for a RAS was designed, including a passive woodchip denitrification followed by a sand filtration introduced into a side-loop of an experimental RAS, rearing rainbow trout (Oncorhynchus mykiss). In the process, woodchips acted as a carbon source for the denitrification, aiming at a simultaneous nitrogen removal and reduction of water consumption while sand filtration was used to remove organic matter and recondition the circulating water. A variety of chemical analyses and toxicological tests were performed to study the suitability of the process and to ensure the absence of harmful or toxic substances in the system. The results did not show increased toxicity, and no increased mortality was reported for the raised species. After the start-up of the system, the concentrations of fatty acids (e.g., hexadecanoic acid −1) and heavy metals (e.g., Cd < LOD-0.45 μg L−1, Pb < LOD-14 μg L−1) remained at very low levels and below those of known toxic effects. In the beginning of the experiment, good denitrification efficiency was achieved, but it declined after 1 month, showing the need for improved stability and dimensioning of the application. [ABSTRACT FROM AUTHOR]

We studied the effect of sand filtration with natural esker material on the removal of total organic carbon (TOC), total suspended solids (TSS), and turbidity from the effluent of an experimental recirculating aquaculture system (RAS) farm. Separate experiments were performed with the same esker sand: (1) a soil column experiment in 2017 where the effluent (mean TOC 8.14 mg L−1) was percolated vertically through a 50-cm-thick sand column with the infiltration 1 m day−1; (2) a sand filtration experiment with water-saturated conditions in 2018 where the effluent from the woodchip denitrification (mean TOC 26.84 mg L−1) was infiltrated through a sand layer with the retention time of 1.2 days. In experiment 2, infiltration of 25 L day−1 through a 31-cm sand layer and 40 L day−1 through a 50-cm sand layer were studied. Both experiments were performed in association with rainbow trout (Oncorhynchus mykiss) grow-out trials. In sand filtration with vertical water flow through a soil column, the removal of TSS was 40%, while of TOC 6%, partly due to the small thickness of the soil column and coarse sand material. In water-saturated conditions, mean removal of TOC (3 mg L−1 1.2 day−1), TSS (1.2 mg L−1 1.2 day−1), and turbidity (0.4 FTU 1.2 day−1) reached 11% (TOC), 18% (TSS), and 15% (turbidity), even with the retention time of only 1.2 days. The removal of TOC in water-saturated conditions correlated with the removal of TSS and turbidity. [ABSTRACT FROM AUTHOR]